Archive | October 2016

Will we all have roofs that double as solar panels in the future?

It is REALLY hard to tell from Elon Musk’s event last week. I know it was mainly about “vision,” but here is a rapid list of the 20 questions we need answers to:

1) What is the exact price per square foot of these tiles? Standard 50 year asphalt roofing shingles cost about $1 per square foot.

2) How much does it cost to install these? Standard roof shingles can be done by anyone with a ladder and basic skills, but even getting a professional to do it with water membrane, etc. will cost you only another $2 per square foot. Installed price is therefore $3/sq. ft. Standard 5kW solar panels are about 400 square feet in area, cost about $15,000 installed, and are therefore $37.50 per square foot. At a guess, a Tesla roof will cost AT LEAST $30,000 installed, assuming that owner put 1,000 square feet on the south facing side of their pitched roof. (More on pitch below.) It could be much more: other solar tiles tend to cost 2X what panels do per square foot. Is this a $50,000 roof?

3) According to the article, “Tesla’s roof will cost less than the full cost of a roof and electricity will be competitive or better than the cost of a traditional roof combined with the cost of electricity from the grid.” We need a lot more data on price assumptions for efficiency, efficiency over time, and electricity costs.

4) Is it much more difficult to install than traditional roofing materials? Most roofers don’t know how to do wiring stuff. Will it be hard? Also, many people need to make changes to their roof over time: install a new vent or skylight or whatever. How does that work with quartz tiles?

5) Solar is one thing, but roofing is another. When a roofer does your roof they don’t just put it up: they offer a warrantee against leakage. How will this work with a novel material requiring different techniques and running electrical connections?

6) If a normal house catches on fire, the fire department cuts through the roof fast to get water inside. Cutting through asphalt is easy, how will they deal with quartz tiles? If the roof is generating 5kW, will firefighters be risking electric shock?

7) In the US, there is a building code rule: 2014 NEC (National Electrical Code), which requires the ability to bring all voltages in the system down below 30V within seconds of shutting the system down. How would this work with Tesla solar tiles?

8) How well do they last? Does their efficiency at generating electricity decline over time? I.e. are they still a good roof after 25 years, but not a good solar panel? This matters a lot, and is also something that no one has the answer for. The ONLY way to really find out what the 25 year efficiency for a new material will be is to install it in real world conditions and wait 25 years!

9) One thing I worry about is heat. If you’ve ever been on a roof on a summer day, it gets HOT up there. Which is a problem for solar panels: they produce less electricity when they get too hot, and if they get too hot too often they become much less good at producing electricity over time. Which is why almost all solar panels are not flush mounted, but have big air gaps behind them to allow the solar cells to cool down. Flush mounted roof tiles might have heat dissipation issues.

10) Many roofs are at an angle/pitch. Facing south would be great for solar, but does it make sense to cover a north facing roof with these tiles? East? West? Whole roof? Part of roof? Does the pitch matter? What about the latitude of the house? Solar panels in Canada are installed at different angles than in Florida, for instance.

11) How durable are they as roof materials? Yes, Elon says they will last a long time, but no-one knows until they have actually been in the field for decades. Would hailstorms crack them?

12) Most roof materials are easily repaired: when one tile or shingle or whatever leaks or breaks, a worker can go up and replace only the damaged portion. Do these tiles work the same way?

13) Also, walking around on a roof is hard already. Glass tiles and any kind of moderate pitch seems pretty slippery. Especially if it is raining at the time. Which, in my experience, is when 100% of all emergency roof repairs occur. 🙂

14) What happens when you get a lot of snow on the roof? (Many inventions that work well in California work much less well in Canada or Sweden!) [Edited to add: according to Musk, you can get tiles with heating elements inside them to melt snow. Which sounds good in one way, but what about the incremental cost, etc?]

15) Exact electrical output, counting latitude, cloud, snow, orientation and roof angles would be nice.

16) Do you have to buy a PowerWall or other battery storage unit? Is that factored into the total cost?

17) Data on water leakage compared to traditional materials over time. At the end of the day, the main point of a roof is to act as a roof. Nobody will care about the electricity generated unless these tiles are at least as effective at keeping the rain out as traditional materials.

18) While we are talking about the environment, can the tiles be recycled or safely disposed of?

19) How much do these things WEIGH? Serious question: all roofs are safe up to a certain weight, but will collapse if they exceed that. This matters especially for snowfall: you can get tons of additional weight up there. People get killed in roof collapses.

20) Elon said that these solar tiles were only 2% less efficient than traditional solar panels. Some people think this means that if the average solar panel is about 20% efficient at converting sunlight to electricity, that the new tiles will be (0.98*20%) or 19.6% efficient, or basically the same. Other people think it means they would be 18% efficient, or 10% less. Big difference. The reason I bring this up is that there have been other solar tiles for years now, and they tend to be about HALF as efficient as standard roof mounted.

Worth noting that last point. There have been other manufacturers of solar roof tiles. Maybe Tesla has solved lot of the problems, but almost all the other solar tile products have left the market. They cost too much, didn’t generate enough electricity, and had many of the other issues I note above.